64 bulletin: museum of comparative zoology. 



organs observed in the early stages, that the whole of these organs 

 is produced by the growth of the ovotesticular fundament of the un- 

 differentiated bud. 



In older buds the genital organs are always distinctly separated from 

 the other organs, and the differentiation of ovary from testis is speedily 

 accomplished. Figures 2 and 3 (Plate i) represent two consecutive 

 cross-sections through a stage in which this separation is going on. In 

 Figure 2, the posterior section, there are two well-defined masses of 

 cells w^hich in Figure 3 are united, the only indication of the future 

 separation being a slight notch on one side of the fundament. The 

 splitting takes place from behind forwards, and appears to be accom- 

 plished by a rearrangement of the cells in situ, and the secretion of a 

 structureless membrana propria between the two masses. Preceding 

 the actual separation, however, there is a differentiation consisting in 

 the fact that the smaller cells, which will produce the testis, are always 

 situated deeper than the others, and this, as Figure 1 shows, is charac- 

 teristic even of the earliest stages. Both ovary and testis are usually 

 solid masses of cells at this stage, and for some time later, but occa- 

 sionally one or both of them may develop a lumen (Plate 1, Fig. 3) , 

 though in the case of the testis it is never well defined. 



Figure 4 (Plate 1) gives a longitudinal section of a somewhat later 

 stage, the ovary and testis being completely separated from each other, 

 but both still attached to a common genital strand {Jun. gen.). The 

 latter extends from the anterior ends of the genital organs to the left 

 peribranchial sac, with the walls of wdiich it fuses. It is much thicker 

 and shorter in the earlier than in the later stages, when the distance 

 that it has to traverse becomes greater. 



An examination of Figures 2, 3, and 4 shows that at this stage there 

 is a well-defined tendency for the long axis of the peripheral nuclei of 

 the testis or testicular part of the ovotestis to occupy a tangential 

 position. This marks the beginning of the peripheral epithelium that 

 bounds the testis at all later stages. In the ovary , however, such a 

 peripheral layer is absent in this stage, as in the earlier ones, so that 

 the young oogonia or their follicles extend to the surface of the organ. 

 This is exactly the condition found by van Beneden et Julin ('85, p. 

 334) for the same stage of these organs in Perophora. Their figures 

 (6 a and 6 h, Planche XVI.) show the peripheral layer well differenti- 

 ated in the testis, but not in the ovary, though the two organs are 

 still connected. 



The fundaments of the oviduct and vas deferens are at first formed 



